Inside of a nuclear fusion reactor e.g. a torus-type nuclear fusion reactor, there is disposed a ring-like vacuum vessel for confining plasma therein. The vacuum vessel is surrounded by toroidal magnetic field coils which produce a magnetic field for keeping the plasma in a confined space. A plurality of the coils are distributed around the vacuum vessel at fixed intervals. A plurality of magnetic field coils are distributed along the vacuum vessel, to produce a magnetic field which heats the plasma and controls the position of the field.
As the materials for constructing the vacuum vessel, non-magnetic alloy steel containing nickel have been proposed. The vacuum vessel of the nuclear fusion reactor is deteriorated by the radiation of the nuclear fusion reaction, e.g. the irradiation of fast neutrons of 14 MeV. It was therefore proposed that Mo plate or W plate etc. of high melting point may be bolted to the water-cooled metal-base body of vacuum vessel as described in (Japanese Patent laid-open No. 55-94181).
But, Mo or W etc. have such large atomic numbers that they are sputtered by plasma particles and permeate the plasma and thus cause a reduction in the temperature of the plasma.
It is very disadvantageous for the nuclear fusion reactor which is exposed to high temperature and cooled with difficulty, that the thermal resistance between water-cooled metal-base body and Mo or W plates bolted thereto is large.